This application is based on Japanese patent application No. Hei 10-130771 filed on May 13, 1998, the whole contents of which are incorporated by reference.
1. Field of the Invention
The present invention relates to a manufacture method of a semiconductor device, more particularly to a manufacture method of a semiconductor device having a diffraction grating.
2. Description of the Related Art
FIG. 6 is a cross-sectional view showing a conventional distributed feedback (DFB) semiconductor laser device. A diffraction grating 110 having a plurality of grooves is formed on a principal surface of an n-type InP substrate 100. A guide layer 101 of n-type InGaAsP is formed on the diffraction grating 110. An active layer 102 of InGaAsP is formed on the guide layer 101. A cladding layer 104 of p-type InP is formed on a guide layer 103. A contact layer 105 of p-type InGaAsP is formed on the cladding layer 104.
An electrode 106 is formed on the contact layer 105. An electrode 107 is formed on a bottom surface of the InP substrate 100.
The active layer 102 and the guide layers 101 and 103 have large refractive indices. Those layers are sandwiched by the InP substrate 100 and the cladding layer 104 which have small refractive indexes to form a waveguide. Only a light having waveform which corresponds to the groove pattern on the diffraction grating 110 is Bragg reflected. The reflections are repeated in the waveguide. As a result, the light is oscillated.
The diffraction grating 110 is formed by the following steps of first forming a resist pattern with the interference exposure, then partially etching the surface region of the InP substrate 100 using the resist pattern as an etching mask.
During the formation of the diffraction grating 110, unevenness of the etching occurs sometimes. In this case, it is difficult to form an even diffraction grating in the substrate. Further, while a substrate is heated for forming the InGaAsP guide layer on the diffraction grating 110, the diffraction grating 110 is occasionally transformed by heat, resulting in the uncontrolled corrugation of the diffraction grating. It is known a method for avoiding such thermal transformation of the diffraction grating 110 by covering the surface of the diffraction grating 110 with GaAs. In this case, however, the crystallinity of the laser structure to be formed on the GaAs layer is easily deteriorated if the condition for covering the GaAs layer is not optimized.
Coupling coefficient, which represents the strength of distributed feedback to the light through the waveguide, depends on the amplitude of the corrugation of a diffraction grating. InP is easily transformed by heat as mentioned above, therefore, it is difficult to control the depth of the grooves on the InP diffraction grating accurately. Moreover, it is also difficult to control the coupling coefficient.
Recently, there is a great demand for a diffraction grating having large coupling coefficient to realize a coolerless DFB laser device. Larger amplitude of the corrugated diffraction grating is necessary for enlarging the coupling coefficient of the diffraction grating. However, if an InGaAsP guide layer is grown on the corrugated diffraction grating of InP having large amplitude, the composition modulation i.e., unevenness of the composition appears. For example, composition of Group III elements on the top of the corrugation differs from that on the bottom of the corrugation. If an active layer is grown on the guide layer having such composition modulation, crystallinity of the active layer is deteriorated.
It is an object of the present invention to provide a manufacture method of a semiconductor device for forming a diffraction grating with high reproductiveness and accuracy, and depositing a high quality semiconductor layer on the diffraction grating.
According to the first aspect of the present invention, it is provided a manufacture method of a semiconductor device comprising the steps of: depositing a III-V compound semiconductor layer, having a refractive index which differs from a refractive index of InP, on a surface of a substrate; forming a plurality of grooves in said compound semiconductor layer so as to reach said substrate surface, in order to form a diffraction grating; forming a first layer of InP so as to fill said grooves and cover said diffraction grating, by metal organic chemical vapor deposition in which an organic metal is used as a source material of In, one of PH3 and organophosphorus is used as a source material of P, and H2 is used as a carrier gas; forming a second layer of InP on said first layer with the temperature of the substrate being higher than that during the first layer formation; and forming an active layer on said second layer; wherein said first layer is deposited at a growth rate lower than such a growth rate of an InP layer as to cause a photoluminescence intensity of a layer corresponding to said active layer to be one tenth as small as that when the InP layer is deposited at a growth rate of 0.2 microns per hour in the case where the InP layer deposition is carried out instead of the first layer deposition under conditions in which the ratio of the flow rate of the source material of P to the total flow rate of the carrier gas and the temperature of the substrate are the same as those during the first layer deposition but the growth rate of the InP layer is different from that of the first layer.
The preferable condition for the first layer""s growth can be found out easily by obtaining the above conditions based on measured photoluminescence intensity of an InGaAsP layer which corresponds to the active layer.
According to the other aspect of the present invention, it is provided a manufacture method of a semiconductor device comprising the steps of: depositing a diffraction grating layer, which is made of a III-V compound semiconductor having a refractive index which is different from a refractive index of InP, on a surface of a substrate; forming a plurality of grooves in said diffraction grating layer so as to reach the substrate surface in order to form a diffraction grating; depositing a first layer of InP so as to fill said grooves and cover said diffraction grating layer, by metal organic chemical vapor deposition in which an organic metal is used as a source material of In, one of PH3 and organophosphorus is used as a source material of P, H2 is used as a carrier gas, and the temperature of the substrate is set at 400-500xc2x0 C., while satisfying an inequality of log10FRxe2x89xa74.4DRxe2x88x921.3 where DR represents the growth rate by microns per hour, and FR % is the ratio of a flow rate of the source material of P to a total flow rate of the carrier gas; and depositing a second layer of InP on said first layer with the substrate temperature being higher than that during the first layer deposition.
After the deposition of the first layer under the above conditions, a III-V compound semiconductor layer having excellent crystallinity can be deposited on the second layer.
According to the other aspect of the present invention, it is provided a manufacture method of a semiconductor device comprising the steps of: depositing a diffraction grating layer, which is made of a III-V compound semiconductor having a refractive index which differs from a refractive index of InP, on a substrate having a surface layer of InP; forming grooves in said diffraction grating layer so as to reach the substrate surface, in order to form a diffraction grating; depositing a first layer of InP so as to fill said grooves and cover said diffraction grating layer, by metal organic chemical vapor deposition in which an organic metal is used as a source material of In, H2 is used as a carrier gas, and the temperature of the substrate is set at 400-500xc2x0 C.; and depositing a second layer of InP on said first layer with the substrate temperature being set higher than that during the first layer deposition; wherein said method further comprises the step of heating said substrate having said grooves up to a temperature for growing said first layer, in H2 atmosphere including the source material of P used in the first layer deposition, before the step of depositing said first layer.
Because the atmosphere during the heating process is the H2 atmosphere including the source material to be P, P is prevented from being removed from the surface of the InP layer.
As explained above, an InP layer is deposited on a diffraction grating of a III-V compound semiconductor under relatively low temperature to cover the diffraction grating. According to this structure, the diffraction grating is prevented from being transformed by heat. After covering the diffraction grating by the InP layer, another InP layer is deposited under relatively high temperature. Thus deposited InP layer shows excellent crystallinity. Because the conditions for the InP layer""s growth under low temperature are optimized, an active layer, a waveguide, and the like having excellent crystallinity can be deposited on the InP layer.